Fuses for protecting electrical circuits are well known. Typically known fuses include an insulating tube or casing which may be made of non-conductive material, such as glass, ceramic or the like. Each of the opposite ends of the tube are closed by a pair of electrically conductive end terminals. An electrically conductive element is provided within the tube, connecting each of the terminals to allow the current to pass therebetween.
In a conventional short circuit fuse, the conductive element is provided with one or more constrictions which overheat and melt almost immediately during a time of a short circuit, producing a high peak reverse voltage. Conventional fuses are disadvantageous in that the gap formed by the melting of the metal at the constriction during overloads may not be sufficiently large to eliminate the arcing of current thereacross.
In prior art dual element fuses, such as time delay or thermally sensitive fuses, an overload protection device is provided within the tube serially connected with the electric conductive element. The simplest type of overload protection device comprises a metal spring held in tension within the fuse with one end secured to a first end terminal, and the other opposite end connected by a heat meltable solder to an end of the electric conductive element. Under prolonged overload conditions, heat generated by the current flow, heats the solder to a temperature where it melts allowing the spring to pull away from the conductive element by its collapse. The collapse of the spring away from the electric conductive element produces a sufficient gap in the current path to break the electric circuit.
A second type of known overload protection device is formed as a preassembly of various parts, which is inserted in series with the conductive element. Known preassemblies typically include a cylindrical housing having disposed therein a plunger in a biased relationship with a compressed or extended helical spring. The plunger extends through and beyond one end of the housing to engage the conductive element. The spring provides a force against the plunger member in a direction away from the conductive element, toward re-entry into the housing. A meltable solder joint is used to electrically connect the end of the plunger to the electric conductive element, thereby preventing the return of the plunger into the housing under the force of the spring.
Upon prolonged overload conditions within the protective circuit, heat generated by a sustained overload current flow through the preassembly and electric conductive element causes the solder joint to melt. Once the solder melts, the plunger member is drawn away from the electric conductive element under the force of the untensioning spring.
One disadvantage of most known overload protection devices is that when positioned within a fuse with filler material, the filler material must be kept from interfering with the operation of the protection device. Typically, this is accomplished by providing dividers to separate the overload protection devices as by forming a separate chamber.
Another disadvantage with fuses incorporating most known overload protection devices is that they are comparatively large, and are therefore unsuitable for use in smaller electrical circuits and the like.
In many fuse applications, the fuse must be of a particular exterior dimension. The fuses contain a number of components to be received within the fuse, frequently in axial alignment. A disadvantage of many fuses is that their individual components are too large to permit advantageous spacing of the components or the inclusion of additional components. In particular, comparatively large sizing of known fuses is required to ensure that on overload activation of the fuse there is provided a sufficient gap in the current path to eliminate arcing of current between the overload protective device and the conductive element. The fuse must be manufactured at least as long as the length of the gap required to break the flow of electricity taken together with the length of the helical spring when compressed.
A further factor attributing to the large size of know time delay fuses is that comparatively large springs are used to ensure there is sufficient force to rapidly and fully move the plunger away from the conductive element on melting of the solder joint.